1. Field of the Invention
The present invention relates to the field of display techniques, and in particular to an AMOLED pixel driver and pixel driving method.
2. The Related Arts
The organic light emitting diode (OLED) display provides the advantages of active light-emitting, low driving voltage, high emission efficiency, quick response time, high resolution and contrast, near 180° viewing angle, wide operation temperature range, and capability to realize flexible display and large-area full-color display, and is regarded as the most promising display technology.
The driving types of OLED can be divided, according to the driving method, into the passive matrix OLED (PMOLED) and active matrix OLED (AMOLED), i.e., the direct addressable type and thin film transistor (TFT) addressable type, wherein the AMOLED provides the advantages of pixels arranged in an array, self-luminous, and high luminous efficiency and is commonly used for high definition large-size display.
AMOLED is a current-driven device that emits light when a current flows through the OLED, and the light-emitting luminance is determined by the current flowing through the OLED. Most of the known integrated circuits (ICs) only transmit voltage signals, so the AMOLED pixel driver circuit needs to complete the task of converting the voltage signal into a current signal.
The known AMOLED pixel driver circuit is usually 2T1C structure, that is, two thin film transistors (TFTs) and a capacitor. As shown in FIG. 1, a known 2T1C pixel driver circuit for AMOLED with a compensation function comprises a first TFT T10, a second TFT T20, a capacitor C10, and an OLED D10, wherein the first TFT T10 has the gate connected to the drain of the second TFT T20, the drain connected to a power supply positive voltage OVDD, and the source connected to the anode of the OLED D10; the second TFT T20 has the gate connected to a gate driving signal Gate, the source connected to a data signal Data and the drain connected to the gate of the first TFT T10; the capacitor C10 has one end connected to the gate of the first TFT T10 and the other connected to the drain of the first TFT T10; the OLED D10 has the anode connected to the source of the first TFT T10 and the cathode connected to the power supply negative voltage OVSS. When the 2T1C-structured AMOLED pixel driver circuit operates, the current flowing through the OLED D10 is:I=k×(Vgs−Vth)2 Wherein I is the current flowing through the OLED D10, k is a constant coefficient related to the characteristics of the first TFT T10, Vgs is the voltage difference between the gate and the source of the driving TFT (i.e., the first TFT T10), Vth is the threshold voltage of the voltage of the driving TFT (i.e., the first TFT T10). As seen, the current flowing through the OLED D10 is related to the threshold voltage of the driving TFT.
Because of the instability of the panel fabrication process, making the threshold voltage of the driving TFT in each pixel drive circuit within the panel maybe different, and the material of TFT will age after prolonged use and result in changes to cause threshold voltage drift of the driving TFT and lead to unstable current flowing through the OLED and causes non-uniformity of panel display. In the known 2T1C circuit, the threshold voltage drift of the driving TFT cannot be improved by adjustment. Therefore, it is necessary to add the new TFT or the new signal to alleviate the influence of the threshold voltage drift, to make the pixel driver circuit have a compensation function.